Layers of rock stack like pages, oldest at the bottom, and radioactive 'clocks' in the minerals tell their age. Deep time is the mind-stretching realization that these pages record billions of years of Earth's diary.
The Earth is 4.5 billion years old
In undisturbed layers of sedimentary rock, the oldest sit at the bottom and the youngest on top — the principle of superposition. This gives relative ages (older/younger). Radiometric dating, using the steady decay of isotopes, gives absolute ages in years.
If Earth's history were one 24-hour day, all of recorded human history would be the last fraction of the final second before midnight.
- Picture a cliff with four rock layers, undisturbed.
- Which layer is oldest?
- A fossil appears in the second layer from the bottom — is it older or younger than one in the top layer?
What you should see: By superposition, the bottom layer is oldest. The lower fossil is older than the top one — relative dating from layer order.